1. Field of the Invention
This invention relates generally to wind turbines, and relates more particularly to a Darrieus-type vertical axis wind turbine having blades fabricated by a pultrusion process.
2. Description of the Relevant Art
A Darrieus-type vertical axis wind turbine ("VAWT") typically has two curved blades joined at the ends to the top and bottom of a rotatable, vertical tower. The blades bulge outward to a maximum diameter about midway between the attachments points at the top and bottom of the tower. See U.S. Pat. No. 1,835,018 to D. J. M. Darrieus for a basic explanation of such a VAWT. The rotatable, vertical tower with the blades attached will be referred to herein as a rotor or rotor assembly. A typical VAWT supports the bottom of the rotor on a lower bearing assembly, which in turn is supported off the ground by a short base. The rotation of the rotor is coupled to and drives an electrical generator, located in the base, that produces electrical power as the rotor rotates. The top of the rotor is supported by an upper bearing assembly that is held in place by guy wires or other structures.
A key component of the VAWT are the blades, which interact with the wind to create lift forces that rotate the rotor and drive the generator. The blades typically have a symmetrical airfoil shape in cross-section with a straight chord that is oriented tangential to the swept area of the turbine. The rotor rotates faster than the wind, and the wind generates lift forces on the blades that maintain rotation of the rotor. The lift forces are periodic because each blade goes through two phases of no lift per revolution when the blade is moving either straight up-wind or straight down-wind. In addition to the wind-generated lift forces, centrifugal forces also act on the blades.
A slender structure like a VAWT blade attached by its ends to a rotating axis tends to take the shape of a troposkein when the rotor rotates. A troposkein is the shape that a linearly-distributed mass like a rope would take under centrifugal force when the rope is spun around an axis. Considering just centrifugal forces, the spinning rope takes the troposkein shape and is loaded in pure tension because it has negligible stiffness or resistance to bending. It is desirable for VAWT blades to have a troposkein shape in order to minimize stresses due to centrifugal forces, but a practical problem is how to fabricate the blades so that they assume a troposkein shape.
Prior blades for VAWTs have typically been made out of extruded aluminum shapes, but these blades are costly and do not form a true troposkein shape. One problem with extruded aluminum blades is that the billet capacities of available extrusion equipment are too small to extrude one-piece blades of sufficient length, which may exceed one hundred feet. Another problem is that forming a one-piece extruded aluminum blade into a curved troposkein shape would cause significant problems in transporting the formed blade to the site of the wind turbine. This is so because the curved blade would be perhaps thirty to forty feet wide and one hundred feet or more long and could not practically be transported by rail or road. As a consequence, an extruded aluminum blade for a VAWT is typically a three-piece assembly with a curved center section and two straight (or partially curved) end sections connecting the ends of the center section to the tower. One disadvantage of such a three-piece assembly is that it only approximates a troposkein and thus generates significant bending stresses, which increases the cost and complexity of the joints joining the center and end sections. Another disadvantage is that aluminum extrusions are costly, and post-extrusion bending adds more cost.
It has been suggested by researchers in the field of wind turbines to use pultruded fiberglass composite blades. Pultrusion is a process where reinforcing fibers coated with resin are pulled through a heated die of a desired cross-sectional shape. The pultrusion process can create extremely long parts of uniform cross-section at low cost. Pultruded blades have been used in horizontal axis wind turbines ("HAWT"), but these blades were straight and, due to the loading and structural requirements of a HAWT, such blades needed to have high stiffness. Pultruded blades have been designed and built for a vertical axis wind turbine, but not of the Darrieus-type. This prior non-Darrieus vertical axis wind turbine with pultruded blades had many straight, vertical blades with curved chords nested together and extending between two horizontal end plates. This wind turbine was similar to the rotor of a squirrel-cage fan oriented with a vertical axis. But again, the blades were straight and required high stiffness. In both these prior pultruded wind turbine blades, it was desirable to have straight blades with high stiffness, but such straight, stiff blades would not work in a Darrieus-type VAWT, which requires curved blades.